1. Field of the Invention
The present invention relates to a mold material composition for preparing a mold (sand mold) and a method for preparing a mold from said mold material composition, and more particularly to a material composition for a self-hardening mold having a good strength particularly after being left standing in the atmosphere at the ordinary temperature and when molten metal is poured therein, a good susceptibility to disintegration or shake-out in spite of the good strength, requiring no mold wash at all or a very simple mold wash such as spraying, and to a method for preparing a mold from said material composition.
2. Related Background Art
A self-hardening mold for preparing a casting sand mold for use in the production of various metal casting products (which will be hereinafter referred to as "mold" or "sand mold") includes an organic binder-based self-hardening mold and an inorganic binder-based self-hardening mold. Known methods based on the organic binder include, for example, a method comprising condensation curing a furan resin as a binder with a strongly acidic hardening agent such as toluenesulfonic acid (Furan method), a method comprising mixing such three components as phenol resin, polyisocyanate and a basic catalyst with molding sand and hardening the phenol resin through the urethanization reaction with the polyisocyanate by the catalyst (Pepset method), a method comprising mixing such three components as an oil-modified alkyd resin, polyisocyanate and a catalyst, for example, a metal naphthenate, with mold sand, followed by hardening through the urethanization reaction (Linocure method), etc.
Known methods based on the inorganic binder include, for example, a method for preparing a mold by solidifying mold sand by cement (OJ process), a method for hardening a sand mold containing sodium silicate by introducing a CO.sub.2 gas thereto under pressure (CO.sub.2 method), etc.
Known mold material compositions for a casting sand mold capable of undergoing casting without any mold wash include, for example, a mold sand composition comprising a furfuryl alcohol condensate, polyester resin, etc.; a hardening agent for curing the resin such as toluene sulfonic acid, etc; a ceramic component such as silicate ester, etc.; an isocyanate-based hardening agent for curing the ester; a fine refractory powder for preventing penetration such as alumina, silica, etc.; a heat resistance intensifier such as sodium chloride, borax, etc.; a molding-improving agent such as sugars, dextrin, etc.; and a graphite speroidization stabilizer such as iron oxide, magnesium oxide, etc. (U.S. Pat. No. 4,775,704), etc.
However, the Furan method uses a strongly acidic hardening agent and thus has such problems as corrosion of casting product surfaces by the remaining acid, slow hardening at the inside of the mold not in contact with air (slow development of mold strength in the deep region), large dependence of hardening time upon the ambient temperature, generation of acidic gas when a molten metal is poured into the mold, resulting in deterioration of working environments, etc. The Pepset method has such problems as a large change in the hardening speed when the kind of sand or ambient temperture is largely changed, and generation of visible smokes (which will be hereinafter referred to as "smokes") when the mold in dismantled. The Linocure method has such problems as somewhat poor mold strength (which will be hereinafter referred to as "mold strength at an elevated temperature") when a molten metal is poured in the mold.
All the molds prepared by using the organic binder generally have poor mold strength at an elevated temperature, and binding forces among the sand grains are decreased due to the combustion of the organic binder when a molten metal is poured into the mold, and surface defects are liable to generate due to penetration of the molten metal into crevices among the sand grains. To prevent the penetration, a mold wash agent containing graphite, quartz powder, etc. as main ingredients must be applied to the parts of a mold in contact with the molten metal.
On the other hand, molds prepared by using the inorganic binder are not susceptible to penetration, but the mold strength after the mold is left standing at the ordinary temperature in the atmosphere for a given time (which will be hereinafer referred to as "mold strength after being left standing") is poor and furthermore a fusion-penetration phenomenon, i.e. melt fusion of the metal onto the mold, is liable to appear. To prevent these adverse phenomena, wood sawdusts, coke powder, etc. must be added to the sand and also a mold wash agent must be applied to the mold.
In any case, a mold wash agent must be used, and the cost of mold wash agent application amounts to 30-50% of the total cost of mold preparation and thus the mold wash agent application is a main cause for mold costup.
In case of the mold material composition for casting sand mold, which is said to be applicable to casting without any mold washing, as disclosed in U.S. Pat. No. 4,775,704, the mold preparation process is very complicated because of use of many binder components such as fine refractory powder for preventing the penetration, a mold-intensifying agent for elevated temperatures etc., and the mold prepared by using these binders has such problems as prolonged hardening time and considerably retarded development of mold strength. Thus, it is practicably difficult to use the mold material composition without any use of mold wash.
Known attempts to reduce the kinds of binder components include, for example, a method using silicate polyol having a hydroxyl value of 200 to 1,500 obtained by reaction of tetraalkoxysilane or its hydrolyzed, dehydrated polycondensate (silicate oligomer) having a hydrolysis ratio of not more than 50% with polyol having two functional groups, a urethanization catalyst, and polyisocyanate (JP-A-Hei3-189048). However, synthesis of silicate polyol is complicated, and the silicate content of the binder component is so low due to the inclusion of silicate into the polyol that no satisfactory mold strength at an elevated temperature can be obtained yet.